Fuel cell stack

ABSTRACT

A fuel cell stack is provided with an outlet side oxygen-containing gas communication hole. First and second oxygen-containing gas flow passage grooves are provided in the direction of the gravity while meandering in the horizontal direction on a surface of a first separator. The second oxygen-containing gas flow passage grooves communicate with the outlet side oxygen-containing gas communication hole via second oxygen-containing gas connecting flow passages. The lowest position of the second oxygen-containing gas flow passage grooves  44  in the direction of gravity is higher than the bottom of the outlet side oxygen-containing gas communication hole  38   b  by the height h.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a fuel cell stack whichcomprises a fuel cell unit composed of a solid polymer ion exchangemembrane interposed between an anode electrode and a cathode electrode,and separators for supporting the fuel cell unit interposedtherebetween, the fuel cell units and the separators being stacked inthe horizontal direction. The fuel cell stack is especially appropriateto be carried on a vehicle.

[0003] 2. Description of the Related Art

[0004] For example, the solid polymer type fuel cell comprises a fuelcell unit including an anode electrode and a cathode electrode disposedopposingly on both sides of an electrolyte composed of a polymer ionexchange membrane (cation exchange membrane) respectively, the fuel cellunit being interposed between separators. Usually, the solid polymertype fuel cell is used as a fuel cell stack obtained by stacking apredetermined number of the fuel cell units.

[0005] In such a fuel cell stack, a fuel gas, i.e., a gas principallycontaining hydrogen (hereinafter referred to as “hydrogen-containinggas”) which is supplied to the anode electrode, hydrogen being convertedinto ion on the catalyst electrode, is moved toward the cathodeelectrode via the electrolyte which is appropriately humidified. Theelectron, which is generated during this process, is extracted for anexternal circuit, and the electron is utilized as DC electric energy. Anoxygen-containing gas such as a gas principally containing oxygen(hereinafter referred to as “oxygen-containing gas”) or air is suppliedto the cathode electrode. Therefore, the hydrogen ion, the electron, andthe oxygen are reacted with each other on the cathode electrode, andthus water is produced.

[0006] In the fuel cell stack described above, an internal manifold isconstructed in order to supply the fuel gas and the oxygen-containinggas (reaction gas) to the anode electrode and the cathode electrode ofeach of the stacked fuel cell units respectively. Specifically, theinternal manifold includes a plurality of communication holes which areprovided in an integrated manner to make communication with each of thefuel cell units and the separators which are stacked with each other.When the reaction gas is supplied to the supplying communication hole,the reaction gas is supplied in a dispersed manner to each of the fuelcell units, while the used reaction gas is integrally discharged to thedischarging communication hole.

[0007] The reaction product water, which is generated on the electrodepower-generating surface, tends to be introduced especially into thecommunication hole through which the oxygen-containing gas flows.Retained water exists in the communication hole in many cases. On theother hand, it is feared that any retained water is generated due tocondensation of water vapor or the like in the communication holethrough which the fuel gas flows. Therefore, the following inconvenienceis pointed out. That is, the communication hole is reduced in crosssectional area or closed by the retained water, and the smooth flow ofthe reaction gas is prevented. As a result, the power generationperformance is deteriorated.

[0008] In view of the above, for example, as disclosed in JapaneseLaid-Open Patent Publication No. 8-138692, a fuel cell is known, inwhich hydrophilic coating films are provided for a fuel gas flow passageand an oxygen-containing gas flow passage formed on a stacking surfaceof a collector electrode. Specifically, as shown in FIG. 28,supply/discharge flow passages 2 a, 2 b for the fuel gas are formed topenetrate through both side portions of a collector electrode 1.Supply/discharge flow passages 3 a, 3 b are formed to penetrated throughupper and lower portions of the collector electrode 1. A plurality ofoxygen-containing gas flow passages 4, which are parallel to one anotherin the vertical direction, are provided linearly on the side of thepower-generating surface of the collector electrode 1. A hydrophiliccoating film 5 is formed for the oxygen-containing gas flow passage 4. Aporous member 6 is arranged for the oxygen-containing gassupply/discharge flow passage 3 b.

[0009] In the arrangement as described above, when the water, which isproduced on the side of the power-generating surface in accordance withthe operation of the fuel cell, is introduced into the oxygen-containinggas flow passages 4, the product water humidifies the hydrophiliccoating film 5 formed for the oxygen-containing gas flow passage 4. Theproduct water flows vertically downwardly along the hydrophilic coatingfilm 5 and its surface, and it is discharged from the oxygen-containinggas flow passage 4. Further, the product water is absorbed by the porousmember 6 which is arranged for the oxygen-containing gassupply/discharge flow passage 3 b. As a result, it is stated that theproduct water can be reliably discharged from the oxygen-containing gasflow passage 4.

[0010] However, in the case of the conventional technique describedabove, the oxygen-containing gas supply/discharge flow passages 3 a, 3 bare formed at the upper and lower portions of the collector electrode 1.Therefore, it is difficult to shorten the size of the entire fuel cellin the height direction. Especially, in the case of the use as a fuelcell stack to be carried on a vehicle, it is necessary to effectivelyutilize the space, e.g., under the floor of the automobile body. It isdemanded to shorten the size of the entire fuel cell in the heightdirection as small as possible. However, the conventional techniquedescribed above involves such a problem that it is impossible toeffectively respond to the demand.

[0011] Further, the oxygen-containing gas supply/discharge flow passages3 a, 3 b are formed to be lengthy in the lateral direction at the upperand lower portions of the collector electrode 1. For this reason, inorder to ensure the rigidity of the collector electrode 1, it isnecessary to set a relatively large thickness of the collector electrode1. Accordingly, the problem is pointed out that the size of the entirefuel cell stack in the stacking direction is lengthy.

[0012] When the size of the entire fuel cell stack in the stackingdirection is lengthy, the oxygen-containing gas supply/discharge flowpassage 3 b becomes long in the stacking direction. The problem alsoarises that the product water or the like existing at the deep side isdifficult to be discharged. Especially, when the fuel cell stack is usedto be carried on the vehicle, it is feared that the vehicle runs in aninclined state, and the product water is retained at the deep portion ofthe oxygen-containing gas supply/discharge flow passage 3 b. At thattime, the problem arises that the power generation performance isdeteriorated because the product water is not discharged.

[0013] A technique is disclosed in Japanese Laid-Open Patent PublicationNo. 10-284096, in which water droplets are prevented from invasion intothe power-generating surface by providing a gas branch groove whichextends downwardly from a gas inlet of a communication hole. However,when the gas branch groove is provided for the power-generating surface,the amount of gas, which is discharged without contributing to the powergeneration, is increased. Then, the problem arises such that the ratioof utilization of the reaction gas is lowered, resulting in the decreasein efficiency of the entire system.

[0014] A technique is disclosed in U.S. Pat. No. 4,968,566, in whichwater discharge ports are provided at lower four corner portions of afuel cell, and the discharge ports are switched depending on a signal ofan inclination sensor. However, the problem arises that the structure ofthe apparatus is considerably complicated.

SUMMARY OF THE INVENTION

[0015] A general object of the present invention is to provide a fuelcell which has a function to smoothly and reliably discharge water, andto prevent water in reaction gas flow passage from freezing.

[0016] The above and other objects, features, and advantages of thepresent invention will become more apparent from the followingdescription when taken in conjunction with the accompanying drawings inwhich a preferred embodiment of the present invention is shown by way ofillustrative example.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 shows a schematic longitudinal sectional view illustratinga fuel cell stack according to a first embodiment of the presentinvention;

[0018]FIG. 2 shows an exploded perspective view illustrating majorcomponents of the fuel cell stack shown in FIG. 1;

[0019]FIG. 3 shows a schematic sectional view illustrating the fuel cellstack shown in FIG. 1;

[0020]FIG. 4 shows a front view illustrating a first separator whichconstitutes the fuel cell stack shown in FIG. 1;

[0021]FIG. 5 shows a front view illustrating a first surface of a secondseparator which constitutes the fuel cell stack shown in FIG. 1;

[0022]FIG. 6 shows a front view illustrating a second surface of thesecond separator;

[0023]FIG. 7 shows a perspective view illustrating a porouswater-absorbing tube and the first separator which constitute the fuelcell stack shown in FIG. 1;

[0024]FIG. 8 shows a perspective view with partial cross sectionillustrating wire members which constitute the porous water-absorbingtube;

[0025]FIG. 9 illustrates a static pressure distribution in the fuel cellstack shown in FIG. 1;

[0026]FIG. 10 shows a perspective view illustrating a porouswater-absorbing tube and a first separator which constitute a fuel cellstack according to a second embodiment of the present invention;

[0027]FIG. 11 shows a partial perspective view illustrating a porouswater-absorbing tube which constitutes a fuel cell stack according to athird embodiment of the present invention;

[0028]FIG. 12 shows a partial perspective view illustrating a porouswater-absorbing tube which constitutes a fuel cell stack according to afourth embodiment of the present invention;

[0029]FIG. 13 shows a longitudinal sectional view illustrating a fuelcell stack according to a fifth embodiment of the present invention;

[0030]FIG. 14 shows a vertical sectional view illustrating a porouswater-absorbing tube which constitutes a fuel cell stack according to asixth embodiment of the present invention;

[0031]FIG. 15 shows a schematic longitudinal sectional view illustratinga fuel cell stack according to a seventh embodiment of the presentinvention;

[0032]FIG. 16 shows an exploded perspective view illustrating majorcomponents of the fuel cell stack;

[0033]FIG. 17 shows a front view illustrating a first surface of abypass plate;

[0034]FIG. 18 shows a sectional view of the bypass plate taken along aline B-B shown in FIG. 17 and an exploded side view of other members;

[0035]FIG. 19 shows a schematic view diagrammatically illustrating afuel cell stack according to a seventh embodiment of the presentinvention;

[0036]FIG. 20 shows a schematic view illustrating a layout of a bypassflow passage according to an eighth embodiment of the present invention;

[0037]FIG. 21 shows a schematic view illustrating a layout of a bypassflow passage according to a ninth embodiment of the present invention;

[0038]FIG. 22 shows a schematic vertical sectional view illustrating afuel cell stack according to a tenth embodiment of the presentinvention;

[0039]FIG. 23 shows a sectional view illustrating an attached state of amanifold tube and a drainage pipe;

[0040]FIG. 24 shows a sectional view illustrating an inserted situationof the drainage pipe;

[0041]FIG. 25 shows a front view illustrating the drainage pipe shown inFIG. 24;

[0042]FIG. 26 shows a part of a separator of a fuel cell according to aneleventh embodiment of the present invention;

[0043]FIG. 27 shows a part of a separator of a fuel cell according to antwelfth embodiment of the present invention; and

[0044]FIG. 28 shows a perspective view illustrating a collectorelectrode concerning the conventional technique.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045]FIG. 1 shows a schematic longitudinal sectional view illustratinga fuel cell stack 10 according to a first embodiment of the presentinvention, and FIG. 2 shows an exploded perspective view illustratingmajor components of the fuel cell stack 10 described above.

[0046] The fuel cell stack 10 comprises a fuel cell unit 12, and firstand second separators 14, 16 for supporting the fuel cell unit 12interposed therebetween. A plurality of sets of these components arestacked with each other. The fuel cell unit 12 includes a solid polymerion exchange membrane 18, and a cathode electrode 20 and an anodeelectrode 22 which are arranged with the ion exchange membrane 18intervening therebetween. First and second gas diffusion layers 24, 26,each of which is composed of, for example, porous carbon paper as aporous layer, are arranged for the cathode electrode 20 and the anodeelectrode 22.

[0047] First and second gaskets 28, 30 are provided on both sides of thefuel cell unit 12. The first gasket 28 has a large opening 32 foraccommodating the cathode electrode 20 and the first gas diffusion layer24. On the other hand, the second gasket 30 has a large opening 34 foraccommodating the anode electrode 22 and the second gas diffusion layer26. The fuel cell unit 12 and the first and second gaskets 28, 30 areinterposed between the first and second separators 14, 16. A thirdgasket 35 is arranged for the second separator 16.

[0048] The first separator 14 is provided, at its upper portions at theboth ends in the lateral direction, with an inlet side fuel gascommunication hole 36 a for allowing a fuel gas (reaction gas) such ashydrogen gas to pass therethrough, and an inlet side oxygen-containinggas communication hole 38 a for allowing an oxygen-containing gas(reaction gas) as a gas containing oxygen or air to pass therethrough.

[0049] The first separator 14 is provided, at its central portions atthe both ends in the lateral direction, with an inlet side coolingmedium communication hole 40 a for allowing a cooling medium such aspure water, ethylene glycol, and oil to pass therethrough, and an outletside cooling medium communication hole 40 b for allowing the coolingmedium after being used to pass therethrough. The first separator 14 isprovided, at its lower portions at the both ends in the lateraldirection, with an outlet side fuel gas communication hole 36 b forallowing the fuel gas to pass therethrough, and an outlet sideoxygen-containing gas communication hole 38 b for allowing theoxygen-containing gas to pass therethrough so that the outlet side fuelgas communication hole 36 b and the outlet side oxygen-containing gascommunication hole 38 b are disposed at diagonal positions with respectto the inlet side fuel gas communication hole 36 a and the inlet sideoxygen-containing gas communication hole 38 a respectively.

[0050] A plurality of, for example, six of mutually independent firstoxygen-containing gas flow passage grooves (gas flow passages) 42, areprovided closely to the inlet side oxygen-containing gas communicationhole 38 a so that they are directed in the direction of the gravitywhile meandering in the horizontal direction on the surface 14 a opposedto the cathode electrode 20 of the first separator 14. The firstoxygen-containing gas flow passage grooves 42 are merged into threesecond oxygen-containing gas flow passage grooves (gas flow passages)44. The second oxygen-containing gas flow passage grooves 44 extend inthe horizontal direction, and terminate at positions close to the outletside oxygen-containing gas communication hole 38 b. The bottom of thesecond oxygen-containing gas flow passage grooves 44 (the lowestposition of the second oxygen-containing gas flow passage grooves 44 inthe direction of gravity) is positioned higher than the bottom of theoutlet side oxygen-containing gas communication hole 38 b by the heighth.

[0051] As shown in FIGS. 2 to 4, the first separator 14 is provided withfirst oxygen-containing gas connecting flow passages 46 which penetratethrough the first separator 14, which communicate at first ends with theinlet side oxygen-containing gas communication hole 38 a on the surface14 b on the side opposite to the surface 14 a, and which communicate atsecond ends with the first oxygen-containing gas flow passage grooves 42on the side of the surface 14 a, and second oxygen-containing gasconnecting flow passages 48 which communicate at first ends with theoutlet side oxygen-containing gas communication hole 38 b on the side ofthe surface 14 b and which communicate at second ends with the secondoxygen-containing gas flow passage grooves 44 on the side of the surface14 a to penetrate through the first separator 14.

[0052] As shown in FIG. 2, an inlet side fuel gas communication hole 36a, an inlet side oxygen-containing gas communication hole 38 a, an inletside cooling medium communication hole 40 a, an outlet side coolingmedium communication hole 40 b, an outlet side fuel gas communicationhole 36 b, and an outlet side oxygen-containing gas communication hole38 b are formed at both end portions in the lateral direction of thesecond separator 16, in the same manner as in the first separator 14.

[0053] As shown in FIG. 5, a plurality of, for example, six of firstfuel gas flow passage grooves (gas flow passages) 60 are formed closelyto the inlet side fuel gas communication hole 36 a on the surface 16 aof the second separator 16. The first fuel gas flow passage grooves 60extend in the direction of the gravity while meandering in thehorizontal direction, and they are merged into three second fuel gasflow passage grooves (gas flow passages) 62. The second fuel gas flowpassage grooves 62 extend in the horizontal direction, and terminate inthe vicinity of the outlet side fuel gas communication hole 36 b. Thebottom of the second fuel gas flow passage grooves 62 (the lowestposition of the second fuel gas flow passage grooves 62 in the directionof gravity) is positioned higher than the bottom of the outlet side fuelgas communication hole 36 b by the height h.

[0054] The second separator 16 is provided with first fuel gasconnecting flow passages 64 for communicating the inlet side fuel gascommunication hole 36 a with the first fuel gas flow passage grooves 60from the side of the surface 16 b, and second fuel gas connecting flowpassages 66 for communicating the outlet side fuel gas communicationhole 36 b with the second fuel gas flow passage grooves 62 from the sideof the surface 16 b to penetrate through the second separator 16.

[0055] As shown in FIGS. 3 and 6, a stepped section 70, whichcorresponds to an opening 68 of the third gasket 35, is formed on thesurface 16 b of the second separator 16. A plurality of main flowpassage grooves 72 a, 72 b for constructing cooling medium flow passagesare formed in the stepped section 70 closely to the inlet side coolingmedium communication hole 40 a and the outlet side cooling mediumcommunication hole 40 b. Branched flow passage grooves 74, which arebranched into a plurality of individuals respectively, are provided toextend in the horizontal direction between the main flow passage grooves72 a, 72 b.

[0056] The second separator 16 is provided with first cooling mediumconnecting flow passages 76 for making communication between the inletside cooling medium communication hole 40 a and the main flow passagegrooves 72 a, and second cooling medium connecting flow passages 78 formaking communication between the outlet side cooling mediumcommunication hole 40 b and the main flow passage grooves 72 b topenetrate through the second separator 16.

[0057] As shown in FIG. 2, an inlet side fuel gas communication hole 36a, an inlet side oxygen-containing gas communication hole 38 a, an inletside cooling medium communication hole 40 a, an outlet side coolingmedium communication hole 40 b, an outlet side fuel gas communicationhole 36 b, and an outlet side oxygen-containing gas communication hole38 b are provided at both end portions in the lateral direction of eachof the first, second, and third gaskets 28, 30, 35.

[0058] As shown in FIG. 1, first and second end plates 80, 82 arearranged at both ends in the stacking direction of the fuel cell units12 and the first and second separators 14, 16. The first and second endplates 80, 82 are integrally tightened and fastened by the aid of tierods 84.

[0059] A porous water-absorbing tube 86 is arranged to extend in thestacking direction for at least the outlet side oxygen-containing gascommunication holes 38 b and optionally for the outlet side fuel gascommunication holes 36 b in the fuel cell stack 10. As shown in FIGS. 1and 7, the porous water-absorbing tube 86 includes a pipe-shaped coremember 88 which is made of metal, for example, SUS (stainless steel),and a plurality of wire members 90 which are wound around the outercircumference of the core member 88.

[0060] As shown in FIG. 8, the wire member 90 has irregularities on thesurface. The respective wire members 90 are bundled, and thus a space 92is formed. The space 92 extends in the longitudinal direction of thecore member 88 (in the stacking direction of the fuel cell stack 10).Both ends of the core member 88 may be closed. The core member 88 isfixed in the fuel cell stack 10 by the aid of an unillustrated fixingmeans.

[0061] As shown in FIGS. 4 and 5, the porous water-absorbing tubes 86are installed on the lower side in the direction of the gravity at thepositions separated from the second oxygen-containing gas connectingflow passages 48 and the second fuel gas connecting flow passages 66, inthe outlet side oxygen-containing gas communication holes 38 b and theoutlet side fuel gas communication holes 36 b.

[0062] As shown in FIG. 1, the first end plate 80 is formed with a hole94 which communicates with the outlet side oxygen-containing gascommunication hole 38 b. A manifold tube 98, which communicates with thehole 94, is connected to the first end plate 80 via a joint 96. Themanifold tube 98 is provided with an outer tube 100 which is bentupwardly from the joint 96. A porous water-absorbing tube 102, which isconnected to the porous water-absorbing tube 86 or which is elongatedfrom the porous water-absorbing tube 86, is arranged in the outer tube100. The porous water-absorbing tube 102 is connected, for example, to awater storage tank (not shown) for storing water which is usable tohumidify and reform the gas.

[0063] The first end plate 80 is formed with a hole 104 whichcommunicates with the outlet side fuel gas communication hole 36 b. Amanifold tube 106, which is constructed in the same manner as themanifold tube 98 described above, is connected to the hole 104, detailedexplanation of which is omitted.

[0064] The operation of the fuel cell stack 10 according to the firstembodiment constructed as described above will be explained below.

[0065] The fuel gas, for example, the gas containing hydrogen obtainedby reforming hydrocarbon is supplied to the inside of the fuel cellstack 10, and the air or the gas containing oxygen as theoxygen-containing gas (hereinafter simply referred to as “air”) issupplied thereto. Further, the cooling medium is supplied in order tocool the power-generating surface of the fuel cell unit 12. As shown inFIGS. 3 and 5, the fuel gas, which is supplied to the inlet side fuelgas communication hole 36 a in the fuel cell stack 10, is moved from theside of the surface 16 b to the side of the surface 16 a via the firstfuel gas connecting flow passages 64. The fuel gas is supplied to thefirst fuel gas flow passage grooves 60 which are formed on the side ofthe surface 16 a.

[0066] The fuel gas, which is supplied to the first fuel gas flowpassage grooves 60, is moved in the direction of the gravity whilemeandering in the horizontal direction along the surface 16 a of thesecond separator 16. During this process, the hydrogen-containing gas inthe fuel gas passes through the second gas diffusion layer 26, and it issupplied to the anode electrode 22 of the fuel cell unit 12. Theunreacted fuel gas is supplied to the anode electrode 22 while beingmoved along the first fuel gas flow passage grooves 60. On the otherhand, the fuel gas, which is not used, passes through the second fuelgas flow passage grooves 62, and it is introduced into the second fuelgas connecting flow passages 66. The fuel gas is moved toward the sideof the surface 16 b, and then it is discharged to the outlet side fuelgas communication hole 36 b.

[0067] As shown in FIG. 3, the air, which is supplied to the inlet sideoxygen-containing gas communication hole 38 a in the fuel cell stack 10,is introduced into the first oxygen-containing gas flow passage grooves42 via the first oxygen-containing gas connecting flow passages 46 whichcommunicate with the inlet side oxygen-containing gas communication hole38 a of the first separator 14. As shown in FIG. 2, the air, which issupplied to the first oxygen-containing gas flow passage grooves 42, ismoved in the direction of the gravity while meandering in the horizontaldirection. During this process, the oxygen-containing gas in the air issupplied from the first gas diffusion layer 24 to the cathode electrode20. On the other hand, the air, which is not used, passes through thesecond oxygen-containing gas flow passage grooves 44, and it isdischarged from the second oxygen-containing gas connecting flowpassages 48 to the outlet side oxygen-containing gas communication hole38 b. Accordingly, the electric power is generated in the fuel cell unit12. For example, the electric power is supplied to an unillustratedmotor.

[0068] Further, the cooling medium, which is supplied to the inside ofthe fuel cell stack 10, is introduced into the inlet side cooling mediumcommunication hole 40 a, and then it is supplied to the main flowpassage grooves 72 a on the side of the surface 16 b via the firstcooling medium connecting flow passages 76 of the second separator 16 asshown in FIG. 6. The cooling medium passes through the plurality ofbranched flow passage grooves 74 which are branched from the main flowpassage grooves 72 a to cool the power-generating surface of the fuelcell unit 12, followed by being merged into the main flow passagegrooves 72 b. The cooling medium after the use passes through the secondcooling medium connecting flow passages 78, and it is discharged fromthe outlet side cooling medium communication hole 40 b.

[0069] During the period in which the fuel cell stack 10 is operated asdescribed above, a relatively large amount of water is producedespecially on the side of the cathode electrode 20. The water isdischarged to the outlet side oxygen-containing gas communication hole38 b via the first and second oxygen-containing gas flow passage grooves42, 44.

[0070] In this case, in the first embodiment, the porous water-absorbingtube 86 is arranged for the outlet side oxygen-containing gascommunication hole 38 b. The water, which is introduced into the outletside oxygen-containing gas communication hole 38 b, is transmitted inaccordance with the capillary phenomenon through the plurality of wiremembers 90 which constitute the porous water-absorbing tube 86. Thewater is introduced into the space 92 formed among the wire members 90.In the fuel cell stack 10, the oxygen-containing gas and the fuel gashave the static pressure distribution as shown in FIG. 9. Accordingly,the pressure on the outlet side of the outlet side oxygen-containing gascommunication hole 38 b is lower than the pressure on the inner side.The water, which is introduced into the space 92 of the porouswater-absorbing tube 86, is extruded by the difference in pressurebetween upper and lower streams of air toward the first end plate 80,i.e., toward the manifold tube 98 as shown by the direction of the arrowA in FIG. 1.

[0071] Accordingly, in the first embodiment, the water, which isintroduced into the outlet side oxygen-containing gas communication hole38 b, is discharged smoothly and reliably toward the porouswater-absorbing tube 102 in the manifold tube 98 owing to the capillaryphenomenon of the porous water-absorbing tube 86 and the difference inpressure of air in the outlet side oxygen-containing gas communicationhole 38 b. According to the simple structure, an effect is obtained thatthe drainage performance for the retained condensed water such as theproduct water is effectively improved.

[0072] Especially, when the fuel cell stack 10 is carried on thevehicle, even if the fuel cell stack 10 is inclined, for example, due toany inclination of the running road, then the water, which is introducedinto the outlet side oxygen-containing gas communication hole 38 b,makes no back flow toward the second oxygen-containing gas flow passagegroove 44. Therefore, the following advantage is obtained. That is, itis possible to prevent the electrode power-generating surface from beingcovered with the product water in the fuel cell stack 10, and it is alsopossible to reliably avoid the deterioration of the power generationperformance.

[0073] Further, as shown in FIG. 4, the porous water-absorbing tube 86is arranged on the lower side in the direction of the gravity of theoutlet side oxygen-containing gas communication hole 38 b at theposition separated from the second oxygen-containing gas connecting flowpassages 48. Accordingly, the water-absorbing performance for theproduct water is improved. It is also possible to avoid the disturbanceof the flow distribution of the air on the side of the electrodepower-generating surface of the first separator. Further, the pressureloss of the air is not increased in the outlet side oxygen-containinggas communication hole 38 b.

[0074] As shown in FIG. 1, the manifold tube 98 is bent upwardly. Theporous water-absorbing tube 102, which is arranged in the manifold tube98, is arranged at the position higher than the outlet sideoxygen-containing gas communication hole 38 b. Accordingly, it ispossible to make the layout of the manifold tube 98 in the plane of thefirst end plate 80. The size of the entire fuel cell stack 10 in theheight direction is not increased. Therefore, the following advantage isobtained. That is, the degree of freedom of the piping layout isimproved. It is possible to effectively shorten the size in the heightdirection of the entire fuel cell stack 10. The fuel cell stack 10 isespecially excellent when it is carried on the vehicle.

[0075] In the first embodiment, as shown in FIG. 2, the inlet side fuelgas communication hole 36 a, the inlet side oxygen-containing gascommunication hole 38 a, the inlet side cooling medium communicationhole 40 a, the outlet side cooling medium communication hole 40 b, theoutlet side fuel gas communication hole 36 b, and the outlet sideoxygen-containing gas communication hole 38 b are provided at the bothend portions in the lateral direction of the fuel cell stack 10.Accordingly, it is unnecessary to provide any lengthy communication holein the lateral direction, at upper and lower portions of the fuel cellstack 10. The size in the height direction of the entire fuel cell stack10 can be as short as possible, and the strength is also improved.Therefore, it is possible to effectively make thinner the size of theentire fuel cell stack 10 in the stacking direction.

[0076] As shown in FIG. 4, the lowest position of the secondoxygen-containing gas flow passage grooves 44 is higher than the bottomof the outlet side oxygen-containing gas communication hole 38 b by theheight h. Thus, even if the water produced in the power generation istrapped in the outlet side oxygen-containing gas communication hole 38b, the water does not flow back into the second oxygen-containing gasflow passage grooves 44. The second oxygen-containing gas flow passagegrooves 44 are not blocked by the water undesirably. The water issmoothly discharged from the second oxygen-containing gas flow passagegrooves 44. In particular, when operation of the fuel cell stack 10 isstarted at a low temperature, water does not freeze in the secondoxygen-containing gas flow passage grooves 44. Therefore, the secondoxygen-containing gas flow passage grooves 44 are not blocked by thefrozen water.

[0077] In the first embodiment, explanation has been made for thosedisposed on the side of the outlet side oxygen-containing gascommunication hole 38 b. The condensed water is similarly produced onthe side of the outlet side fuel gas communication hole 36 b as well. Itis possible to provide the efficient and reliable drainage function byusing the porous water-absorbing tube 86. Although the porouswater-absorbing tube 86 has the pipe-shaped core member 88, it is alsopreferable to use a rod-shaped member alternatively.

[0078] In the first embodiment, the first and second oxygen-containinggas flow passage grooves 42, 44, which are the gas flow passage grooves,are provided in the direction of the gravity while meandering in thehorizontal direction on the surface 14 a of the first separator 14. Onthe other hand, the first and second fuel gas flow passage grooves 60,62, which are the gas flow passages, are provided in the direction ofthe gravity while meandering in the horizontal direction on the surface16 a of the second separator 16. Alternatively, the respective gas flowpassages may be provided in the direction of the antigravity whilemeandering in the horizontal direction on the surfaces 14 a, 16 a of thefirst and second separators 14, 16. In this arrangement, the porouswater-absorbing tubes 86 are arranged on the upper side of the first andsecond separators 14, 16. However, the same effect is obtained, forexample, that the drainage performance for the retained condensed wateris effectively improved owing to the capillary phenomenon of the porouswater-absorbing tube 86 and the difference in pressure of air or thelike. The pattern of the gas flow passages is not limited to theserpentine pattern. Alternatively, straight gas flow passages may besuitably employed depending on the application.

[0079]FIG. 10 shows a perspective view illustrating a porouswater-absorbing tube 120 and a first separator 14 which constitute afuel cell stack according to a second embodiment of the presentinvention. The same constitutive components as those of the fuel cellstack 10 according to the first embodiment are designated by the samereference numerals, detailed explanation of which will be omitted.Explanation will be made in the second embodiment and the followingdescribed below in the same manner as described above.

[0080] The porous water-absorbing tube 120 comprises a pipe member 122made of metal, for example, SUS, and a plurality of wire members 124accommodated in the pipe member 122. The pipe member 122 has a pluralityof holes 126 on the outer circumference. The water can be penetratedthrough the holes 126 into the pipe member 122. The surfaceconfiguration of the wire member 124 is irregular in the same manner asin the wire member 90.

[0081]FIG. 11 shows a partial perspective view illustrating a porouswater-absorbing tube 130 which constitutes a fuel cell stack accordingto a third embodiment of the present invention, and FIG. 12 shows apartial perspective view illustrating a porous water-absorbing tube 140which constitutes a fuel cell stack according to a fourth embodiment ofthe present invention.

[0082] The porous water-absorbing tube 130 has a large number of holes132, comprising an angular pipe member 134 having a square crosssection, and a plurality of wire members 136 arranged in the angularpipe member 134. The porous water-absorbing tube 140 has a large numberof holes 142, comprising a triangular pipe member 144 having atriangular cross section, and a plurality of wire members 146 arrangedin the triangular pipe member 144. The angular pipe member 134 and thetriangular pipe member 144 are arranged along the shape of the cornersof the outlet side oxygen-containing gas communication hole 38 b and theoutlet side fuel gas communication hole 36 b.

[0083] In the porous water-absorbing tubes 120, 130, 140 constructed asdescribed above, the water permeates through the respective holes 126,132, 142. The same effect as that obtained in the first embodiment isobtained. That is, for example, it is possible to discharge the watersmoothly and reliably owing to the capillary phenomenon of the pluralityof wire members 124, 136, 146 and the difference in pressure of air.

[0084]FIG. 13 shows a longitudinal sectional view illustrating a fuelcell stack 160 according to a fifth embodiment of the present invention.In the fuel cell stack 160, porous water-absorbing tubes 162 arearranged for the outlet side oxygen-containing gas communication hole 38b and the outlet side fuel gas communication hole 36 b. The porouswater-absorbing tube 162 comprises a pipe member 164 and a plurality ofwire members 166 arranged at the inside of the pipe member 164.

[0085] The pipe member 164 is provided with a plurality of holes 168which are formed at portions to be arranged at the outlet sideoxygen-containing gas communication hole 38 b and the outlet side fuelgas communication hole 36 b, making it possible to effect the permeationof water. On the other hand, no hole is provided at the portion exposedto the outside of the fuel cell stack 160. The pipe member 164 isconstructed in an integrated manner. However, it is also preferable thata tube provided with the holes 168 and a tube having no hole areseparately provided, and they are fixed by means of a joint or the like.It is also preferable to use a variety of water-absorbing members inplace of the wire member 166.

[0086]FIG. 14 shows a vertical sectional view illustrating a porouswater-absorbing tube 180 which constitutes a fuel cell stack accordingto a sixth embodiment of the present invention. The porouswater-absorbing tube 180 comprises a water-absorbing member 182 having asolid cross section which is embedded on the lower side in the directionof the gravity of each of the outlet side oxygen-containing gascommunication hole 38 b and the outlet side fuel gas communication hole36 b. The water-absorbing member 182 is composed of, for example,sponge. The upper surface of the water-absorbing member 182 is set atthe position to provide a predetermined gap S from the secondoxygen-containing gas connecting flow passage 48 and the second fuel gasconnecting flow passage 66. It is possible to avoid any back flow ofwater. If the water-absorbing member 182 is not used, the bottom of theoutlet side oxygen-containing gas communication hole 38 b and the bottomof the outlet side fuel gas communication hole 36 b are spaced from thesecond oxygen-containing gas connecting-flow passage 48 and the secondfuel gas connecting flow passage 66 by the height h, respectively. Thus,the back flow of water is prevented, and the water does not freeze inthe gas flow passages.

[0087]FIG. 15 shows a schematic longitudinal sectional view illustratinga fuel cell stack 200 according to a seventh embodiment of the presentinvention.

[0088] The fuel cell stack 200 comprises a bypass plate 202 which istightened and fastened by the tie rods 84 together with the second endplate 82. As shown in FIGS. 16, 17, and 18, the bypass plate 202 isarranged in tight contact with the surface 14 b of the first separator14 which is disposed on the deep side of the inlet sideoxygen-containing gas communication hole 38 a and the outlet sideoxygen-containing gas communication hole 38 b of the fuel cell stack200, i.e., on the deep side as viewed from a supply port K of the inletside oxygen-containing gas communication hole 38 a and a discharge portH of the outlet side oxygen-containing gas communication hole 38 b asshown in FIG. 19 as described later on. An inlet side oxygen-containinggas communication hole 38 a of the bypass plate 202 is provided at theposition corresponding to the inlet side oxygen-containing gascommunication hole 38 a of the first separator 14.

[0089] Discharge holes 204 for supplying the oxygen-containing gas to beused for the reaction, which are provided for the bypass plate 202, aredisposed at positions corresponding to a bottom T2 of the outlet sideoxygen-containing gas communication hole 38 b of the first separator 14in a direction directed along the outlet side oxygen-containing gascommunication hole 38 b of the first separator 14.

[0090] As shown in FIGS. 16 and 17, bypass flow passages 208, which makecommunication between the outlet holes 204 and a plurality of inletholes 206 having rectangular cross sections formed at the hole wall ofthe inlet side oxygen-containing gas communication hole 38 a of thebypass plate 202, are provided on the surface 202 b of the bypass plate202 on the side opposite to the surface 202 a to make tight contact withthe first separator 14.

[0091] The bypass flow passages 208 includes three groove-shaped supplypassages 208 a which makes connection over a range from the inlet sideoxygen-containing gas communication hole 38 a to the discharge holes 204at diagonal positions, and two auxiliary supply passages 208 b whichextend from the inlet holes 206 along the upper and lower supplypassages 208 a of the supply passages 208 a and which are merged intothe supply passages 208 a at intermediate positions.

[0092] As shown in FIG. 17, as for the inlet side oxygen-containing gascommunication hole 38 a of the bypass plate 202, the bottom T1, whichprovides an opening position of the lowermost inlet hole 206, is set ata position (also shown in FIG. 18) that is lower by a height Δh1 thanthe inlet side oxygen-containing gas communication hole 38 a of thefirst separator 14. The discharge hole 204 is located at the bottom T2of the outlet side oxygen-containing gas communication hole 38 b of thefirst separator 14. The discharge hole 204 is opened at a position whichis lower by a height Δh2 than the position of the lowermost secondoxygen-containing gas connecting flow passage 48.

[0093] In order to avoid the back flow, the upper two of the supplypassages 208 a are bent at positions just before the discharge holes204, and they are formed and directed downwardly. The auxiliary supplypassages 208 b are also bent at positions just before the merging pointsP of the supply passages 208 a, and they are formed and directeddownwardly.

[0094] The bypass plate 202 constructed as described above is tightenedand fastened by the tie rods 84 together with the end plate 82 as shownin FIG. 15 in a state in which a terminal plate 210 and an insulatingplate 212 are allowed to intervene as shown in FIG. 18. Therefore,strictly speaking, the bypass flow passages 208 are formed between thebypass plate 202 and the terminal plate 210.

[0095] The bypass flow passages 208 are designed so that theoxygen-containing gas flows in a flow rate which is not less than a flowrate of the flow of the oxygen-containing gas through the fuel cell unit12.

[0096] As schematically shown in FIG. 19, in the fuel cell stack 200,the deep portion of the inlet side oxygen-containing gas communicationhole 38 a and the deep portion of the outlet side oxygen-containing gascommunication hole 38 b are connected to one another by the bypass flowpassages 208 formed, for example, by the bypass plate 202 and the endplate 82. Accordingly, a return flow structure is formed, in which thesupply port K of the inlet side oxygen-containing gas communication hole38 a as the side to supply the oxygen-containing gas and the dischargeport H of the outlet side oxygen-containing gas communication hole 38 bas the side to discharge the gas after the reaction are provided on thesame side, i.e., on the first side surface side of the fuel cell stack200. In this arrangement, the single bypass plate 202, which has thethin plate-shaped configuration, is used. Therefore, the structure isadvantageous in that no piping is required at the outside of the fuelcell stack 200. It is possible to shorten the size of the fuel cell unit12 in the stacking direction.

[0097] The supply port K and the discharge port H are disposed on thesame side. Therefore, a merit is obtained such that the piping for thesupply port K and the discharge port H can be provided as collectedpiping which is advantageous in the number of assembling steps and thenumber of parts.

[0098] In the seventh embodiment constructed as described above, theproduct water tends to stay on the deep side of the outlet sideoxygen-containing gas communication hole 38 b as compared with the frontside. However, a part of the oxygen-containing gas, which is supplied tothe inlet side oxygen-containing gas communication hole 38 a, passesfrom the inlet hole 206 of the bypass plate 202 through the bypass flowpassages 208 as shown in FIG. 19, and it is ejected at the dischargehole 204 to the outlet side oxygen-containing gas communication hole 38b. Accordingly, the product water, which is retained at the deep portionof the outlet side oxygen-containing gas communication hole 38 b, isextruded toward the discharge port H. This arrangement is also preferredespecially in the case of application to the vehicle which runs in aninclined state.

[0099] In this arrangement, as shown in FIG. 17, the auxiliary supplypassages 208 b are merged into the supply passages 208 a of the bypassflow passages 208. Therefore, the flow rate is increased at thedischarge hole 204. It is possible to efficiently extrude the productwater at the discharge hole 204. In this arrangement, the position ofthe discharge hole 204 (position of the bottom T2 of the outlet sideoxygen-containing gas communication hole 38 b) is set to be lower by Δh2than that of the second oxygen-containing gas connecting flow passage48. Therefore, there is no fear of back flow.

[0100] On the other hand, the water is generated in some cases due tothe condensation of water vapor at the inlet side oxygen-containing gascommunication hole 38 a, because the oxygen-containing gas ishumidified. However, the position of the inlet hole 206 of the bypassflow passage 204 (position of the bottom T1 of the inlet sideoxygen-containing gas communication hole 38 a of the bypass plate 202)is set to be lower by Δh1 than that of the bottom of the inlet sideoxygen-containing gas communication hole 38 a, for example, of the firstseparator 14 and the first gasket 28. Therefore, it is also possible toefficiently discharge the condensed water.

[0101] Therefore, in the seventh embodiment, the drainage performance isremarkably improved for the retained product water and the condensedwater, and it is possible to avoid the deterioration of the powergeneration performance, by forcibly extruding the oxygen-containing gasfrom the discharge hole 204 by the aid of the bypass flow passage 202,in addition to the improvement in drainage performance owing to thecapillary phenomenon of the porous water-absorbing tube 86 and thedifference in pressure of air in the outlet side oxygen-containing gascommunication hole 38 b.

[0102] The bypass flow passage 202 used in the seventh embodimentdescribed above may be incorporated into any one of the second to fifthembodiments.

[0103]FIG. 20 diagrammatically shows a fuel cell stack 220 according toan eighth embodiment of the present invention. Specifically, FIG. 20shows another embodiment of the bypass flow passage 202.

[0104] The eighth embodiment is different from the seventh embodiment asshown in FIG. 19. The supply port K of the inlet side oxygen-containinggas communication hole 38 a and the discharge port H of the outlet sideoxygen-containing gas communication hole 38 b are provided on differentsides, i.e., on the sides of the opposing surfaces of the fuel cellstack 220.

[0105] In the eighth embodiment, the supply port K is arranged on thesame side as the deep side of the outlet side oxygen-containing gascommunication hole 38 b. Therefore, the oxygen-containing gas for theinlet side oxygen-containing gas communication hole 38 a is branchedwith a branched tube 222, and it is supplied to the outlet sideoxygen-containing gas communication hole 38 b.

[0106] Accordingly, also in this case, the product water, which isretained at the deep portion of the outlet side oxygen-containing gascommunication hole 38 b, can be extruded from the discharge port H.During this process, when the condensed water is retained at the deepportion and the front portion of the inlet side oxygen-containing gascommunication hole 38 a, the condensed water on the side of the inletside oxygen-containing gas communication hole 38 a can be also smoothlydischarged by providing a bypass passage 224 as shown by dashed lines inFIG. 20.

[0107]FIG. 21 diagrammatically shows a fuel cell stack 230 according toa ninth embodiment of the present invention. Even in the case of thearrangement of the inlet side oxygen-containing gas communication hole38 a and the outlet side oxygen-containing gas communication hole 38 bin the same manner as in FIG. 20, a piping 232 can be detoured toconnect the deep portion of the inlet side oxygen-containing gascommunication hole 38 a and the front portion of the outlet sideoxygen-containing gas communication hole 38 b. Also in this case, in thesame manner as in the eighth embodiment described above, it is possibleto reliably discharge the product water at the deep portion of theoutlet side oxygen-containing gas communication hole 38 b and thecondensed water at the inlet side oxygen-containing gas communicationhole 38 a.

[0108] The present invention is not limited to the respectiveembodiments described above. For example, in place of the provision ofthe bypass plate 202 of the seventh embodiment, a bypass piping forconnecting the inlet side oxygen-containing gas communication hole 38 aand the outlet side oxygen-containing gas communication hole 38 b may beprovided at the outside of the second end plate 82. The respectiveembodiments are illustrative of the case in which the discharge hole 204and the porous water-absorbing tube are simultaneously used. However,the porous water-absorbing tube may be abolished, and only the dischargehole 204 may be provided, provided that the product water or the like issufficiently discharged. Alternatively, a plurality of discharge holes204 may be provided in the length direction of the outlet sideoxygen-containing gas communication hole 38 b to extrude the productwater from an intermediate portion as well as the deep portion as viewedfrom the side of the discharge port H of the outlet sideoxygen-containing gas communication hole 38 b.

[0109]FIG. 22 shows a schematic sectional view illustrating a fuel cellstack 240 according to a tenth embodiment of the present invention.

[0110] Reference numeral 242 indicates a terminal plate, and referencenumeral 244 indicates an insulator plate. The terminal plate 242 and theinsulator plate 244 are overlapped with the second separator 16 in anorder of the terminal plate 242 and the insulator plate 244 on the sideof the first end plate 80, and they are tightened and fastened togetherwith the first end plate 80 by means of the tie rods 84. On the otherhand, the terminal plate 242 and the insulator plate 244 are overlappedwith the first separator 14 in an order of the terminal plate 242 andthe insulator plate 244 on the side of the second end plate 82, and theyare tightened and fastened together with the second end plate 82 bymeans of the tie rods 84.

[0111] Each of the terminal plate 242 and the insulator plate 244 on theside of the first end plate 80 is also provided, at both end portions inthe lateral direction, with an inlet side fuel gas communication hole 36a, an inlet side oxygen-containing gas communication hole 38 a, an inletside cooling medium communication hole 40 a, an outlet side coolingmedium communication hole 40 b, an outlet side fuel gas communicationhole 36 b, and an outlet side oxygen-containing gas communication hole38 b. The first end plate 80 is formed with a hole 94 which communicateswith the outlet side oxygen-containing gas communication hole 38 b. Amanifold tube 98, which communicates with the hole 94, is connected tothe first end plate 80 by the aid of a joint 246.

[0112] The manifold tube 98 includes an outer tube (gas flow passage)100 which extends from the joint 246. The outer tube 100 rises upwardly,and it is open to the atmospheric air. A back pressure valve 248 foradjusting the pressure in the fuel cell stack 240 is provided at anintermediate position of the outer tube 100. A drainage pipe (suctionmember) 250 is inserted into the outlet side oxygen-containing gascommunication hole 38 b and the hole 94 of the first end plate 80 in astate of making no contact with the surroundings.

[0113] As shown in FIGS. 23 to 25, the drainage pipe 250 is applied to aresin coating for the outer wall in order to ensure the insulationperformance with respect to the inner wall of the outlet sideoxygen-containing gas communication hole 38 b. A resin end cap 252 isattached to the end of the insertion side in order to ensure theinsulation performance as well. The drainage pipe 250 is supported in astate of being inserted into a fitting hole 242 a which is formed forthe terminal plate 242 disposed on the side of the second end plate 82.

[0114] A suction hole (opening) 254, which is open in the outlet sideoxygen-containing gas communication hole 38 b, is provided at the lowerwall at the end on the insertion side of the drainage pipe 250, i.e., onthe lower side in the direction of the gravity. A stay plate 256 isattached to the outer circumference at the end on the first end plate 80of the drainage pipe 250. The stay plate 256 is positioned with respectto the first end plate 80, and it is tightened together with the joint246. Therefore, a merit is obtained such that the stay plate 256 can beused to correctly set the drainage pipe 250 in the outlet sideoxygen-containing gas communication hole 38 b. The end of the drainagepipe 250 on the side of the first end plate 80 slightly protrudes fromthe first end plate 80.

[0115] A bypass tube (outlet side flow passage) 258 is connected to thedrainage pipe 250 formed as described above. As shown in FIG. 22, thebypass tube 258 has one end which is connected to the drainage pipe 250,and the other end which is connected to the downstream side of the backpressure valve 248. As shown in FIG. 23, a female connector 258 a isattached to the one end of the bypass tube 258. The drainage pipe 250 isfitted and connected to the female connector 258 a. The female connector258 a is inserted and fixed into the joint 246 in a state of penetratingthrough the joint 246. A seal ring S for sealing the drainage pipe 250is provided on the inner circumferential surface of the female connector258 a.

[0116] A ring member 260, which is provided with a seal ring S on theouter circumferential surface, is attached in the vicinity of theattachment portion of the bypass tube 258 to the female connector 258 a.The ring member 260 is inserted in a sealed state into the innercircumferential surface of a pipe support holder 262 which is providedaround the insertion section of the outer tube 100. The joint 246 of themanifold tube 98 constructed as described above is tightened andfastened to the first end plate 80 together with the stay plate 256 bythe aid of bolts B.

[0117] A hole 104, which communicates with the outlet side fuel gascommunication hole 36 b, is formed for the first end plate 80. Amanifold tube 106, which is constructed in the same manner as themanifold tube 98 described above, is connected to the hole 104. Thedrainage pipe 250 is inserted into the hole 104. The bypass tube 258 isconnected in communication with the drainage pipe 250. These structuresand the like are the same as those of the outlet side oxygen-containinggas communication hole 38 b. The same components are designated by thesame reference numerals, detailed explanation of which will be omitted.

[0118] As shown by chain lines in FIG. 22, a throttle section 264 isprovided at the inside of the outer tube 100 at the connecting portionof the bypass tube 258 connected to the downstream side of the backpressure valve 248. Accordingly, an ejector section 266 can be formed bythe bypass tube 258 and the throttle section 264 as well.

[0119] The operation of the fuel cell stack 240 according to the tenthembodiment constructed as described above will be explained below.

[0120] During the operation of the fuel cell stack 240, the pressure inthe system is adjusted to be constant by the aid of the back pressurevalve 248. Therefore, a certain differential pressure is generatedbetween the upstream side and the downstream side of the back pressurevalve 248. The difference in pressure is generated by the differentialpressure between the side of the first end plate 80 and the side of theback pressure valve 248 of the bypass tube 258. As a result, the productwater in the outlet side oxygen-containing gas communication hole 38 bis sucked from the suction hole 254 of the drainage pipe 250. Theproduct water passes through the bypass tube 258, and it is dischargedtogether with the reacted gas from the manifold tube 98 to the outsideof the system.

[0121] Similarly, for example, the product water, which is generated bycondensation of the water vapor of the fuel gas stored in the outletside fuel gas communication hole 36 b, is also sucked from the suctionhole 254 of the drainage pipe 250. The product water passes through thebypass tube 258, and it is discharged together with the reacted gas fromthe manifold tube 98 to the outside of the system.

[0122] Accordingly, when the product water or the like is retained inthe outlet side oxygen-containing gas communication hole 38 b and theoutlet side fuel gas communication hole 36 b, then the product water canbe sucked from the suction hole 254 of the drainage pipe 250, and it canbe discharged to the outside. Therefore, it is possible to improve thedrainage performance for the product water or the like retained in theoutlet side oxygen-containing gas communication hole 38 b and the outletside fuel gas communication hole 36 b, and it is possible to avoid thedeterioration of the power generation performance, which would beotherwise caused by the back flow of water droplets into thepower-generating surface. That is, the water is discharged by thesuction based on the differential pressure not by the gravity.Therefore, it is possible to perform the drainage quickly and reliably.

[0123] As a result, even when the water is retained on the deep side ofthe outlet side oxygen-containing gas communication hole 38 b and theoutlet side fuel gas communication hole 36 b disposed on the sideopposite to the first end plate 80, the water can be reliablydischarged. Therefore, this arrangement is preferred when the fuel cellstack 240 is used for the vehicle in which the fuel cell stack 240 isused in an inclined state.

[0124] Further, the product water, which tends to be retained on thelower side by the gravity, can be sucked and removed by using thedrainage pipe 250 from the outlet side oxygen-containing gascommunication hole 38 b and the outlet side fuel gas communication hole36 b disposed on the lower side in the direction of the gravity.Therefore, it is possible to efficiently discharge the water from thelower portions in the direction of the gravity in which the back flow tothe power-generating surface is apt to occur, and a large amount ofproduct water or the like is retained. In this arrangement, it ispossible to reliably discharge the water from the outlet sideoxygen-containing gas communication hole 38 b and the outlet side fuelgas communication hole 36 b by the aid of the suction hole 254 formed onthe lower side in the direction of the gravity.

[0125] Further, the back pressure valve 248, which is provided for theoutlet side oxygen-containing gas communication hole 38 b and the outletside fuel gas communication hole 36 b, is effectively utilized to makeit possible to suck and remove the product water retained in the outletside oxygen-containing gas communication hole 38 b and the outlet sidefuel gas communication hole 36 b from the suction hole 254 of thedrainage pipe 250. Therefore, it is unnecessary to provide anyadditional pump or the like, and is possible to simplify the structure.As described above, when the ejector section 264 is provided, it ispossible to perform the drainage more effectively, because the force fordrawing the water by the drainage pipe 250 is increased by the ejectoraction.

[0126] In the fuel cell stack 240 described above, the piping isattached to only one side of the end plate. Therefore, the followingeffect is obtained. That is, it is possible to decrease the pipingspace, it is possible to use the collective piping, and thus the pipingstructure is simplified.

[0127] In the tenth embodiment described above, for example, thedrainage pipe 250 described above may be also provided for the inletside fuel gas communication hole 36 a and the inlet sideoxygen-containing gas communication hole 38 a. In this arrangement, itis possible to avoid any invasion of water droplets into thepower-generating surface from the inlet side fuel gas communication hole36 a and the inlet side oxygen-containing gas communication hole 38 a.

[0128] The tenth embodiment is illustrative of the case in which thedrainage pipe 250 is provided for the outlet side oxygen-containing gascommunication hole 38 b and the outlet side fuel gas communication hole36 b. However, for example, it is also preferable that the drainage pipe250 is provided for only the side of the outlet side oxygen-containinggas communication hole 38 b in which a large amount of product water isretained. The position of the suction hole 254 formed for the drainagepipe 250 is not limited to the end portion on the side of the second endplate 82. The suction hole 254 may be provided at the end portion on theside of the first end plate 80. In this arrangement, even when the fuelcell stack 240 is inclined toward either the first end plate 80 or thesecond end plate 82, it is possible to discharge the water from eithersuction hole 254 in a reliable manner. Of course, a plurality of suctionholes may be provided over an entire region of the insertion section ofthe drainage pipe 250.

[0129] The formation portion is not limited to the outer circumferentialedge portion provided that the inlet side fuel gas communication hole 36a, the inlet side oxygen-containing gas communication hole 38 a, theoutlet side fuel gas communication hole 36 b, and the outlet sideoxygen-containing gas communication hole 38 b are formed in the planesof the first and second separators 14, 16.

[0130]FIG. 26 shows a part of a first separator 28 of a fuel cellaccording to an eleventh embodiment of the present invention.

[0131] The first separator 280 has a buffer section 284 between the endof the second oxygen-containing gas flow passage grooves 44 and theoutlet side oxygen-containing gas communication hole 38 b. A pluralityof bosses 282 are formed in the buffer section 284. The bottom of thesecond oxygen-containing gas flow passage grooves 44 is spaced from thebottom of the outlet side oxygen-containing gas communication hole 38 bby the height h. Thus, the water is smoothly discharged into the outletside oxygen-containing gas communication hole 38 b, and is not trappedbetween the bosses 282 and the bottom of the second oxygen-containinggas flow passage groves 44. The buffer section 284 is not essentialdepending on the application. If the buffer section 284 is not provided,the second oxygen-containing gas flow passage grooves 44 are directlyconnected to the outlet side oxygen-containing gas communication hole 38b.

[0132]FIG. 27 shows a part of a first separator 290 of a fuel cellaccording to a twelfth embodiment of the present invention.

[0133] The first separator 290 has the similar structure as the firstseparator 280 according to the eleventh embodiment, but differs in thatthe bottom of the outlet side oxygen-containing gas communication hole38 b is cut out to form a dent section 292 having a narrow bottom. Thebottom of the second oxygen-containing gas flow passage grooves 44 isspaced from the narrow bottom of the dent section 292 in the outlet sideoxygen-containing gas communication hole 38 b by the height h. Thus, thewater is even more smoothly discharged from the second oxygen-containinggas flow passage grooves 44 into the outlet side oxygen-containing gascommunication hole 38 b.

[0134] In the fuel cell stack according to the present invention, thecommunication holes for allowing the reaction gas to flow are providedat the outer circumferential edge portions on the side of the separator.Accordingly, it is possible to shorten the size in the height directionas small as possible, and it is easy to realize the thin type. Further,the water in the communication hole can be discharged smoothly andreliably owing to the capillary phenomenon and the difference inpressure of the reaction gas by the aid of the porous water-absorbingtube arranged in the communication hole. Accordingly, even when the fuelcell stack is arranged in an inclined manner due to any inclination ofthe vehicle or the like, then the back flow of the water to the gas flowpassage can be effectively excluded, the power generation performancecan be ensured, and it is possible to greatly improve the drainageperformance with the simple structure.

[0135] According to the present invention, even if the product water orthe like is retained in the outlet side communication hole, when thereaction gas is supplied from the discharge hole, then the product wateror the like, which is retained in the outlet side communication hole, isextruded by the reaction gas ejected from the discharge hole. Therefore,it is possible to improve the drainage performance for the product wateror the like retained in the outlet side communication hole, especiallythe drainage performance for the product water or the like which isretained at the deep portion and which is difficult to be discharged.Accordingly, it is possible to enhance the drainage performance for theproduct water or the like even when the size of the entire fuel cellstack in the stacking direction is long.

[0136] Further, according to the present invention, when the productwater or the like is retained in the inlet side communication hole orthe outlet side communication hole, then the product water can be suckedfrom the opening of the suction member, and it can be discharged to theoutside. Therefore, it is possible to improve the drainage performancefor the product water or the like retained in the inlet sidecommunication hole or the outlet side communication hole. As for theinlet side communication hole, it is possible to avoid any invasion ofwater droplets into the power-generating surface. As for the outlet sidecommunication hole, it is possible to avoid any back flow of waterdroplets into the power-generating surface. Thus, it is possible toavoid the deterioration of the power generation performance.

What is claimed is:
 1. A fuel cell formed by stacking a fuel cell unitand separators in a horizontal direction such that said fuel cell unitis sandwiched between said separators, said fuel cell unit including ananode, a cathode, and a solid polymer electrolyte membrane interposedbetween said anode and said cathode, communication holes extendingthrough only horizontal ends of said separators, said communicationholes including a first communication hole extending through onehorizontal end of said separator at an upper position, and a secondcommunication hole extending through the other horizontal end of saidseparators at a lower position; a reaction gas flow passage formed on atleast one of said separators, said reaction gas flow passage having aninlet connected to said first communication hole, and an outletconnected to said second communication hole such that said reaction gasflows along an electrode surface in the horizontal direction, wherein abottom of said outlet of said reaction gas flow passage is positionedhigher than a bottom of said second communication hole.
 2. The fuel cellaccording to claim 1, wherein said reaction gas flow passage comprises aplurality of grooves, and a buffer section is provided between saidgrooves of said reaction gas flow passage and said second communicationhole.
 3. The fuel cell according to claim 1, wherein said reaction gasflow passage comprises a plurality of grooves, and said grooves of saidreaction gas flow passage are directly connected to said secondcommunication hole.
 4. The fuel cell according to claim 1, wherein saidreaction gas flow passage comprises a plurality of grooves, and saidflow grooves of said reaction gas flow passage are connected to one endsof connection holes on one surface of said separator, and the other endsof said connection holes are connected to said second communication holeon the other surface of said separator.
 5. The fuel cell according toclaim 1, wherein said reaction gas flow passage comprises a continuousgroove connecting said first communication hole and said secondcommunication hole.
 6. The fuel cell according to claim 5, wherein saidflow groove is a serpentine groove.
 7. The fuel cell according to claim5, wherein said flow groove is a straight groove.
 8. The fuel cellaccording to claim 1, wherein said reaction gas flow passage is anoxygen-containing gas flow passage.
 9. The fuel cell according to claim1, wherein said fuel cell is configured to be mounted in a vehicle. 10.The fuel cell according to claim 1, wherein a dent section having anarrow bottom is formed on said second communication hole.